(1) Field of the Invention
The present invention relates to a gas rate sensor for maintaining especially gas temperature constant.
(2) Description of the Prior Art
Conventional gas sensors contain a holder assembly having a flow sensor in a hermetic, metallic casing and a gas flow is injected towards a thermosensitive element in the flow sensor to detect the angular velocity (the rate) on the basis of variation in the sensor output produced when the gas flow is deviated by the effect of the angular velocity movement exerted from the outside. Because variation in the ambient temperature may, in this case, exert adverse effect on the detection precision, a proposal has been offered to wind heater wires around the outside of the casing containing the holder assembly to maintain the casing and the inside thereof at a constant temperature.
FIG. 6 illustrates a structural diagram showing the main portion (main body) of a conventional gas rate sensor provided with heater wires in this type. In this drawing, a numeral 1 denotes a holder assembly containing a flow sensor and the like in the inside and a numeral 2 denotes a metallic casing containing the holder assembly 1 with the inside of the casing being hermetic. Heater wires 3 are helically wound around the outside of the casing 2 through the intermediary of an insulator and a thermister 4 which is a temperature detection element is attached to the outer surface of the casing 2 by means of an adhesive. Thermosensitive elements 5a and 5b and a piezo plate 6 constituting the flow sensor are held in the holder 1a of the holder assembly 1. Gas from an exhaust port 7 passes, as given in an arrow mark in the drawing, through a portion between the holder assembly 1 and the casing 2 to define a gas flow passage from a nozzle 8 at the end of the holder assembly 1 to the inside of the holder.
In the main body of the gas rate sensor thus constructed, inside of the casing being hermetic, a constant gas flow is injected from the nozzle 8 towards the thermosensitive elements 5a and 5b of the flow sensor and is deviated when the effect of the angular velocity movement is exerted from the outside. In this case, the degree of the angular movement can be detected by detecting variation in the output of the flow sensor. Conduction of the heater wires 3 is controlled by the detection signal of the thermister 4 and the temperatures of and in the casing 2 are maintained constant by means of the feedback control whereby the casing 2 acts as a thermostat. For this reason, even if the ambient temperature has varied, no effect is exerted on the holder assembly 1 whereby a high degree of detection precision can be obtained.
Because the above gas rate sensor is merely so constructed that the holder assembly 1 is inserted into the casing 2, however, the contact area of the holder 1a with the casing 2 is small and temperature control is performed while detecting the temperatures of the casing 2 and the heat wires 3 by means of the thermister 4. Accordingly, there are problems in that the temperature of the structure in the holder assembly 1 slowly rises and stabilizes, resulting in poor detection characteristics and the fixing member of the holder assembly 1 is required.
Meanwhile, known cited references are U.S. Pat. Nos. 3,587,328 dated June 28, 1971, 3,628,371 dated December 21, 1971, 4,020,699 dated May 3, 1977, 4,020,700 dated May 3, 1977 and 4,026,159 dated May 31, 1977.